Nickel and iron based superalloys are regularly employed in jet engine components because of their excellent physical and mechanical properties, particularly at elevated temperatures. For example, it is not uncommon for a turbine blade within a jet engine to be exposed to operating temperatures near or even above 2000.degree. F. Therefore, it is essential at these harsh operating conditions that these materials have exceptional strength as well as excellent corrosion resistance. To achieve these desired characteristics, these superalloys are alloyed with aluminum and titanium. In addition to strengthening the nickel and iron based alloys, these alloying elements form tenacious oxides on the surface of the component which provide the material with the necessary oxidation protection.
Brazing is a common manufacturing procedure used to join two or more surfaces. Brazing is used routinely to join individual components made from these nickel and iron based superalloys for formation of more complex jet engine assemblies. Generally, these materials are brazed together by providing an appropriate braze alloy between the mating surfaces and then heating the materials to a temperature sufficient to melt the braze alloy without melting the surrounding metals. The braze alloy melts at a lower temperature because it typically contains a melting point suppressant, such as boron. As the melting point suppressant diffuses away from the brazed region into the base metal, the remaining braze metal solidifies at the brazing temperature. The resulting structure is characterized by a permanent metallurgical bond between the braze alloy and surrounding materials.
Unfortunately there are inherent difficulties associated with the brazing methods used to join superalloy materials. Although the protective aluminum and titanium oxides within the superalloys are necessary for oxidation resistance, they become a hindrance during brazing of the superalloys. These oxides prevent the braze filler alloy from sufficiently wetting the surface of the surrounding material. Therefore, the braze filler alloy does not sufficiently flow onto the surrounding material. This impedes uniform and complete brazing between the two surfaces.
As a solution to this problem, the regions of the superalloy material which are to be brazed, are nickel plated prior to the brazing operation. This is accomplished by first appropriately masking the superalloy component, and then electrochemically plating nickel onto those regions which are to be brazed and which have not been masked. The nickel plating improves the brazeability of the superalloys by preventing oxide formation in those regions which are to be bonded, thereby enhancing the wetting action of the braze alloy. However, there are shortcomings associated with the nickel electroplating process of these materials.
Nickel plating is unduly time consuming and expensive as it necessitates several additional processing steps for the masking, plating and subsequent baking operations required. Further, these problems are magnified when the plated component is complex in shape and geometry, thereby making it extremely difficult to mask the desired areas. Consequently if the nickel plated region or mask are faulty, the resulting brazed joint between mating parts may be defective. Therefore there is a strong need for a brazing means for these superalloys which does not require the surface of the superalloy to be first nickel plated.
In addition, conventional methods for applying the braze filler alloy to the desired region to be brazed are to use a slurry paste, foil or wire form of the braze alloy. The braze alloy is applied to the desired region and the components are assembled. However during assembly of the components, the bond between the braze alloy and surrounding materials is minimal since it is based primarily on the surface tension between the materials, thereby making dislodgment of the braze alloy a possibility. This is particularly troublesome when assembling jet engine components where dimensional tolerances are frequently within the thousandths of an inch. Therefore, it would be advantageous if the braze alloy could be specifically and permanently applied to the superalloy material prior to assembly of the components.
Accordingly, what is needed is a means for brazing these superalloys wherein the braze filler alloy can be specifically applied and metallurgically bonded to the desired region of the base metal prior to the brazing operation, so as to alleviate the need for nickel plating techniques.